A typical long-range optical transmission system includes a pair of unidirectional optical fibers that support optical signals traveling in opposite directions. An optical signal is attenuated over long distances. Therefore, the optical fibers typically include multiple repeaters that are spaced apart from one another. The repeaters include optical amplifiers that amplify the incoming, attenuated optical signals. The repeaters also include an optical isolator that limits the propagation of the optical signal to a single direction.
In long-range optical transmission systems it is important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the fiber optic cable such as attenuation in the optical fiber and splice loss, faulty repeaters or amplifiers or other problems with the system. Optical time domain reflectometry (OTDR) is one technique used to remotely detect faults in optical transmission systems. In OTDR, an optical pulse is launched into an optical fiber and backscattered signals returning to the launch end are monitored. In the event that there are discontinuities such as faults or splices in the fiber, the amount of backscattering generally changes and such change is detected in the monitored signals. Since backscatterring and reflection also occurs from elements such as couplers, the monitored OTDR signals are usually compared with a reference record, new peaks and other changes in the monitored signal level being indicative of changes in the fiber path, normally indicating a fault. The time between pulse launch and receipt of a backscattered signal is proportional to the distance along the fiber to the source of the backscattering, thus allowing the fault to be located. In a WDM system, one wavelength is usually assigned as the OTDR channel.
FIG. 1 shows a simplified block diagram of a wavelength division multiplexed (WDM) transmission system that employs a conventional OTDR. Each terminal 110 and 120 includes an OTDR unit 105 and 107, respectively. The transmission path consists of unidirectional optical fibers 106 and 108 that support optical signals traveling in opposite directions. In operation, OTDR unit 105 generates an optical pulse that is launched into optical fiber 106. The optical pulse serves as the OTDR probe signal. Because optical isolators 115 located downstream from each optical amplifier 106 prevent the OTDR probe signal from being reflected and backscattered to the OTDR 105 on fiber 106, each repeater 114 includes a coupler arrangement providing an optical path for use by the OTDR. In particular, signals generated by reflection and scattering of the probe signal on fiber 106 between adjacent repeaters enter coupler 118 and are coupled onto the opposite-going fiber 108 via coupler 122. The OTDR signal then travels along with the data on optical fiber 108. OTDR 107 operates in a similar manner to generate OTRD signals that are reflected and scattered on fiber 108 so that they are returned to OTDR 107 along optical fiber 106.
Clearly, in the OTDR arrangement shown in FIG. 1, an OTDR unit is required for each of the two unidirectional optical paths. As additional unidirectional optical paths are added between the terminals of a WDM transmission system, yet even more OTDR units will be required.